This invention relates to a 1.3 .mu.m-band amplifying optical fiber preform of an optical fiber which is used for a fiber optical amplifier in optical communication systems.
There is an increasing demand to directly amplify light signals in optical communication systems. For this purpose, fiber optical amplifiers have been recently developed. As compared with electrical amplification in which light signals are converted into electrical signals, fiber optical amplifiers which directly amplify light signals by stimulated emission have superior characteristics such as higher gain, lower noise, broader bandwidth, compactness of the product size and lower price.
In optical communication systems, 1.3 .mu.m-band and 1.5 .mu.m-band are mainly used. With respect to 1.5 .mu.m-band, the practical applicability of fiber optical amplifiers has been demonstrated with using a erbium doped silica glass fibers. 0n the other hand, development of fiber optical amplifiers operating at 1.3 .mu.m-band is still under way. Hitherto, various oxide glass fibers such as silica glass fiber, which are doped with rare earth ions such as neodymium ion (Nd.sup.3+) as an active ion have been proposed to be used for fiber optical amplifiers operating at 1.3 .mu.m-band. However, these fibers do not exhibit a sufficiently high gain at 1.3 .mu.m-band, because of the presence of strong excited state absorption (ESA). Thus, fluoride glass fibers have been proposed to be used for fiber optical amplifiers operating at 1.3 .mu.m-band. Fluoride glass fibers can suppress ESA and get a sufficiently high gain. In fluoride glass fibers, it is conventional to use fluoride glass for both of a core and a cladding of the fiber. It is usual to prepare a preform of this type of fluoride glass fiber by build-in casting method [see Phys. Chem. Glasses, 23(6), 196 (1982)] or by rotational casting method [see Electron. Lett., 18, 657 (1981)] because fluoride glasses are more easily crystallized and low in weatherability as compared with common oxide glasses.
However, in the case that both of the cladding and the core are made from fluoride glass, it is difficult to get the above range of the diametral ratio by decreasing diameter of the core through the above-mentioned build-in casting method or the rotational casting method. The reason of this is that the minimum diameter of the core which can be produced by the above methods is several millimeters. Furthermore, in the case that both of the cladding and the core are made from fluoride glass, it is difficult to get the above range of the diametral ratio by increasing diameter of the cladding. The reason of this is that fluoride glass is easy to be crystallized. Therefore, there is a certain limit in enlargement of the size of the cladding. There is another proposal to increase diameter of the cladding by jacketing of fluoride glass tube. However, the thus formed cladding is insufficient in mechanical strength.
It is usual to widen specific refractive index difference between the cladding and the core by adding specific elements or by varying glass composition. However, in the case that both of the cladding and the core are made from fluoride glass, it is difficult to sufficiently widen the difference because fluoride glass tends to be crystallized by doping and by varying composition.